Mommy has a "right" in most places to breastfeed but she does not have the right to have FAs even ask other passengers to move, and especially to move from paid Y+ seats. If she's uncomfortable breastfeeding in public, that is her problem, not yours. Perhaps the FAs could have offered her the use of jumpseats in the rear of the aircraft if she didn't want to use the lavatory for this purpose and to avoid having her monopolize a lavatory for long periods.

Since these were last minute seat assignments, I wonder whether the family even paid for Y+ or otherwise was entitled to it. They also apparently got an empty seat as it seemed to be mother, father, and infant rather than three paid tickets for seats.